1. Field of the Invention
The present invention relates to a honeycomb filter and a urea SCR device.
2. Discussion of the Background
Toxic gases such as NOx contained in exhaust gases discharged from internal combustion engines, for example, vehicles such as buses and tracks, construction machines, and the like, and particulate matter (hereinafter, also referred to simply as “PM”) have raised problems as contaminants harmful to the environment and the human body.
In order to solve the above problems, there has been developed a urea SCR (Selective Catalytic Reduction) device including, as a main component, a honeycomb structure (hereinafter also referred to as catalyst carrier) in which a NOx conversion catalyst is supported for converting NOx in exhaust gases (for example, WO02/38922A1).
The urea SCR device includes a catalyst carrier, a holding sealing material which is wound around the periphery of the catalyst carrier for supporting the catalyst carrier, and a casing for placing therein the catalyst carrier and the holding sealing material. The urea SCR device also includes a urea spray nozzle for spraying urea water disposed anterior to the region through which exhaust gases flow into the catalyst carrier (at an upstream side of exhaust gases).
The catalyst carrier has a pillar shape and is made of a porous ceramic. Also, the catalyst carrier has a large number of cells placed in parallel with one another each extending in a longitudinal direction from one end to the other end of the catalyst carrier with a cell wall interposed therebetween. The cell wall supports a zeolite as a NOx conversion catalyst. The ends of the cells are not sealed with a sealing material, and thus each cell is penetrating from one end to the other end of the catalyst carrier.
In conversion of toxic gases with the use of the urea SCR device, urea water is sprayed to the catalyst carrier. This causes thermal decomposition of the urea contained in the urea water by the heat of the exhaust gases to generate ammonia.
When exhaust gases containing NOx are introduced to the cells from one end of the catalyst carrier, NOx in the exhaust gases is reduced to N2 by the effect of the zeolite supported on the cell wall and the ammonia and thus converted.
The exhaust gases in which NOx has been converted are discharged from the other end side of the catalyst carrier.
On the other hands, in order to purify PM in exhaust gases, a various kinds of honeycomb structures (diesel particulate filter (hereinafter also referred to simply as DPF)) in which either one end of each cell is sealed have been proposed (for example, JP-A 2008-272737).
The DPF has a pillar shape and is made of a porous ceramic. Also, in the DPF, a large number of cells are placed in parallel with one another each extending in a longitudinal direction from one end to the other end of the DPF with a cell wall interposed therebetween. Either one end of each cell is sealed with a sealing material.
Therefore, exhaust gases flowing into the cells which are open on one end pass through the cell walls between the adjacent cells, and are then discharged from the cells which are open on the other end. As the exhaust gases pass through the cell walls, PM contained in the exhaust gases is captured by the cell walls so that the exhaust gases are purified.
When the captured PM is accumulated to a predetermined amount and thus the pressure loss reached a given value, a regeneration process for heating a DPF is performed. As a result, PM is burned away and the DPF is regenerated.
The contents of WO02/38922A1 and JP-A 2008-272737 are incorporated herein by reference in their entirety.